(a) Field of the Invention
This invention relates to lead anodes for electrowinning metals from sulfuric acid solutions and to a method of manufacturing such anodes.
(b) State of the Art
Lead anodes have been used for years in electrowinning of copper, nickel, zinc, and other metals. In the use of lead alloys for electrowinning of metals from sulfuric acid solutions, the lead becomes an insoluble, stable anode. The property of lead which accounts for this use is the ability of lead to form an insoluble corrosion film which can repair itself if damaged and prevent further corrosion of the lead anode. In sulfuric acid, an initial thin lead sulfate corrosion layer is converted via the applied current to lead dioxide by anodization. The oxygen generated at the anode during electrowinning reacts with the lead to form lead dioxide and converts lead sulfate to lead dioxide. For optimum performance of the anode, the alloy should form a thin, hard, dense, compact, adherent layer of lead dioxide on the surface. Such a layer will not spall off, deteriorate or contaminate the cathode product.
Conventional lead anodes are cast to shape with the cast lead convering a copper bus bar. This method of attachment uses excess amounts of lead, produces a wide anode because of the lead covering over the copper bus bar, and often gives poor contact between the lead and copper bar. In addition, since the lead must flow around the bar in casting, dross and air are often trapped in the area of the bus bar limiting conductivity and giving potential areas for corrosion or shorting. A conventional method of anode manufacturing is described in U.S. Pat. No. 4,124,482.
An anode of wrought lead-calcium-tin alloy in sheet form has also been employed in recent years for electrowinning metals from sulfuric acid solutions. Such sheet anodes have simply been bolted or otherwise mechanically attached to the bus bar.
A new improved means for attaching a metal sheet to a bus bar has now been discovered. The resulting anode has a uniform, smooth transition joint between the bus bar and sheet material and thus exhibits better conductivity and greater corrosion resistance than conventionally cast or mechanically fastened lead anodes. Moreover, the anodes of the invention can be of thinner construction than conventional anodes.